1. Field of the Invention
The invention is in the field of integrated optical circuits. More particularly, it relates to an integrated optical waveguide circuit for splitting an optical input signal into two output signals with optical powers in a predetermined splitting ratio.
2. Prior Art.
Splitting optical signals is one of the most important functions in an optical system. For optical distribution systems of, for example, TV signals from a sending location to a large number of receivers optical 1.fwdarw.N splitters are known, which often are composed of optical 3 dB splitters. A 3 dB splitter is a power splitter in which the optical power of an optical input signal is evenly distributed over two output signals, having therefore a splitting ratio of 50/50. For optical signal monitoring or in optical control loops often smaller fractions, for example 10%, are split out of the optical power of an optical signal. Depending on the physical principle applied, known splitters can roughly be distinguished into two kinds. A first kind is based on interference. Splitting circuits in which a direction coupling device or an MZ-interferometer is applied belong to this kind. Although said circuits are of a relatively short length, they require well-defined phase-relations between the different optical signals and consequently a narrow manufacturing tolerance. Furthermore, they are polarisation and wavelength dependent and it is often difficult to realise specific splitting ratios having a sufficient accuracy. A second splitter kind is based on symmetry. The type most generally known of this kind is the symmetric Y splitter which possesses a mono-modal waveguiding trunk, branching symmetrically into two mono-modal wave guiding branches. Due to its symmetry, it principally is a 3 dB splitter. A second type of said second kind, for example, is known through references [1] and [2]. Said second type relates to an asymmetric Y splitter having a mono-modal trunk, which widens adiabatically to a bi- or multi-modal waveguide and subsequently splits into two mono-modal branches having different propagation constants at a relatively large splitting angle. In this connection, the coupling area adjacent to the splitting point between the two branches forms a disruption, as a result of which partial conversions in propagation modes are caused, in an optical signal entering via the trunk, from the zero order to the first order. The signals which have different orders of propagation mode subsequently further propagate via the different branches. The extent of conversion consequently determines the splitting ratio and, inter alia, depends on the widths chosen for the two branches and on the size of the angle between both branches. A splitting point which to a large degree reflection-free is attained by having the effective refraction index in the propagation direction decrease gradually in the area between both branches starting from the splitting point. Although this known asymmetric Y splitter is polarisation-independent to a high degree and to some extent wavelength-independent, a desired specific splitting ratio cannot be realised easily. A third splitter type of the second kind, with which a well-defined fraction can be split out of an optical signal, is known through reference [3]. Said splitter is a combination of a mode converter and an asymmetric Y splitter used as a modus splitter. In this combination, the asymmetric Y splitter has a bi-modal wave guiding trunk which splits at a relatively small angle into two mono-modal branches having different propagation constants and is incorporated in the bi-modal trunk of a mode-converter. Depending on the magnitude of the order of the propagation mode of an input signal in the bi-modal trunk, the light mainly couples in the one or the other of both branches. As a result of a specific periodical structure of the mode converter, of an optical signal, a desired fraction of a zero propagation mode is converted into a first propagation mode. Although said known splitter has a splitting ratio which can be defined well, as a consequence of the interferometric nature of the mode converter it is selective, to a high extent, for wavelength and polarisation of the optical signal used.